1. Field of the Invention
The present invention generally relates to the field of radio communication technology and, more particularly, to high throughput millimeter-wave point-to-point radio relay communication systems. The present invention represents a system and method of relay communication with electronic beam adjustment. The present invention provides both automatic initial alignment of antenna beam directions of two radio relay stations as well as further adjustment of beam directions to compensate for small antenna orientation changes caused by various factors (such as wind, vibration, different expansion coefficients of the bearing structures at temperature fluctuations, etc).
2. Description of Related Art
Point-to-point radio relay systems are widely used in various transport networks for multiple applications, the most important being backhauling of base station sites for mobile networks. Recent growth of data traffic served by the mobile networks leads to ever growing requirements to the capacity of radio interfaces of the mobile networks and, as a consequence, to the capacity of the exploited backhaul links. The requirements to the capacity of the backhaul for each cellular site are expected to grow in the next several years from the current several Mbps (or even hundreds of Kbps) to hundreds of Mbps or even several Gbps per site.
The growing throughput requirements for radio relays create the necessity for expanding the bandwidth of transmission signal to accommodate extra capacity. Since the available spectrum of frequency bands in use is typically exhausted, exploitation of higher carrier frequency and new spectrum bands is needed. In particular, the millimeter-wave spectrum with the frequencies from 30 GHz to 300 GHz has significant potential for high-capacity backhaul applications and the 71-76 GHz and 81-86 GHz bands have been made available in many countries for fixed point-to-point radio links with no licensing or simplified licensing.
The increase in the operational frequency and signal bandwidth significantly impacts the design of radio backhaul links. The received signal power per Hz of the signal bandwidth needs to be kept the same in comparison with a lower band system to maintain the same spectral efficiency. This leads to a requirement for higher total received power that can be achieved by rising the transmit power or by increasing the gain of transmit and/or receive antennas. The transmit power is typically limited by the regulations that leaves the only option of increasing the antenna gain. At the same time, the well-known Friis transmission equation (H. T. Friis, Proceedings of IRE, vol. 34, p. 254, 1946):
            P      r              P      t        =            G      t        ⁢                            G          r                ⁡                  (                      λ                          4              ⁢              π              ⁢                                                          ⁢              R                                )                    2      shows that the received power Pr in free space (that is a valid approximation for a point-to-point radio relay link) is proportional to the gains of the receiver Gr and transmitter Gt antennas, transmit signal power Pt and the squared wavelength λ of the carrier frequency and inversely proportional to the squared propagation distance R. Thus moving to a higher frequency band even at the same signal bandwidth also requires increasing the antenna gain at the transmitter and/or receiver in order to maintain the link budget.
A related parameter to the antenna gain is the antenna half power beam width (HPBW) that decreases when the antenna gain grows. Due to the above discussed reasons, typical antenna gains for the millimeter-wave radio relay systems are 40-50 dBi that translates to antenna beam widths in the order of 1 degree and below.
The use of antennas with such narrow beam widths in the radio relay systems leads to additional problems in their operation not occurring in antenna systems with lower carrier frequency and bandwidth. The outdoor units of the relay stations including their antennas are typically mounted close to the base stations on high height constructions such as cell towers or rooftops of high buildings. The cell towers are typically metal constructions that are relatively rigid but still have some swings and deviations (especially of their top part) caused by wind, different expansion coefficients of the bearing structures due to heating/cooling in different hours of a day and other reasons. High buildings are also subject to the same problems (though at a less extent) because of low drifts of their bases that leads to small changes of the building orientations especially closer to the roof level.
Such changes of the orientations of the mounting constructions may lead to loss of the mutual orientation of the transmitter and receiver antennas and failure of the communication link. This problem became now acute for millimeter-wave radio relay systems where the beam widths of the used antennas are about 1 degree or below because of the reasons considered above.
The frequency and typical time scales of the mounting construction orientation variations are very different depending on the cause of the variation and may vary from a fraction of Hz (or even several Hz) for wind induced oscillations, hours for variations due to different heating by the sun of different parts of the construction and up to the years for low drifts of the building bases.
The current attempts to reduce or eliminate these drawbacks include mounting relay systems to lower level structures of cell towers (not on the top) and further to rooftops of buildings and, additionally, manually adjusting antenna's direction in case of slow drifts of their orientations.
Relay Systems for Mobile Networks Backhauling
Relay systems are widely used for mobile networks backhauling. In comparison with traditional fiber optical communication links, radio relay systems benefit from significantly lower installation costs, competitive throughput that has become recently comparable to the throughput of traditional systems. The above mentioned benefits stimulate further research and development of the theoretical and practical aspects of designing efficient millimeter-wave transceivers.
Thus, at present various point-to-point and point-to-multipoint communication systems and modifications thereof for mobile networks backhauling are proposed. In particular, U.S. Pat. No. 7,769,347 titled “Wireless communication system” describes a wireless communication system that provides data communication between a plurality of mobile users with the help of a base station network. Herewith communication links between the base stations are established directly or by the additional communication points in the millimeter-wave band higher than 60 GHz. It should be noted that the authors of the invention pay the most attention to the 71-76 GHz and 81-86 GHz frequency bands which advantages were considered in the current description earlier. It is also mentioned that antenna for the relay station should have narrow beam width and high gain.
U.S. Pat. No. 7,769,347 teaches the importance of antenna beam fine adjustment and describes some methods of beam adjustment.
Adjustment methods disclosed therein include mechanical scanning of different reflector antennas and electronic beam scanning in phased antenna arrays. The use of phased antenna arrays is however impractical nowadays for the millimeter-wave relay systems due to the very large antenna elements number requirement. The above mentioned modern methods of mechanical scanning include rotation of the whole antenna using precise mechanisms, movement or rotation of primary antenna elements of the reflectors, movement of the secondary comparatively small reflector of, for example, Cassegrain antenna, usage of multi-aperture antennas together with the array of switched antenna elements and are far from being optimal. All methods of mechanical scanning are comparatively slow and can require the manual work using qualified staff.
Relay stations are also disclosed, for example, in US Patent Application No 20080153549 titled “Wireless millimeter wave communication system”. In particular, this patent application describes full duplex transceiver for relay system operating in the considered frequency bands of 71-76 GHz and 81-86 GHz wherein the antenna is a lens antenna with a primary horn feed antenna element. Utilization of the lens antenna in this case provides lower side lobe level of the antenna radiation pattern comparing with the traditional for millimeter-wave relay stations reflector antennas. The problem of rough and fine mutual antenna beam adjustment is noted in the application description but no new solutions are suggested.
In the prior art, multiple methods for rough and fine antenna beam direction adjustment are disclosed. Rough adjustment is performed using some simple but precise optical devices, for example, an optical or laser sight. Fine adjustment may be performed by one of various proposed methods. Some of these methods include additional equipment and devices such as a power detector for received signal power estimation, other methods use common operation mode of the transceiver. For example, a method disclosed in U.S. Pat. No. 7,501,982 entitled “Antenna alignment method” is based on maximization of the link budget depending on the antenna orientation. It is further noted in the description of U.S. Pat. No. 7,501,982 that antenna rotation is performed either manually by the personnel or by a special motor device.
Systems and Methods for Remote Antenna Adjustment
One proposed solution to the problem of antenna beam positioning is remote antenna beam direction adjustment and tracking, which is advantageous in many cases. Remote adjustment allows performing antenna beam adjustment and tracking in automatic manner without long connection failure.
A remote antenna positioning system is disclosed in U.S. Pat. No. 7,642,961 entitled “Remote control antenna positioning system”. The positioning system described therein is based on the use of electrical precise motor for antenna rotation in 3-D space and is controlled remotely. However, this system requires the use of expensive precise motor technique and a computer system for signal processing controlled by a qualified operator.
Another prior art solution of a method of electronic beam adjustment is disclosed in U.S. Pat. No. 6,587,699 entitled “Narrow beamwidth communication link with alignment camera”. This prior art describes a point-to-point communication system that consist of two separated high throughput transceivers with highly directional antennas, each transceiver is equipped with a telescopic camera, and a special processor for processing of the received from cameras images (see FIG. 1). The system described in U.S. Pat. No. 6,587,699 provides not only fine mutual antenna orientation adjustment but also automatic beam directions tracking and tuning in cases of shifts or swings of the relay station mounting construction. The main concept of this system for beam adjustment is receiving some reference images from cameras for which the best signal transmission characteristics are provided. When the beam is deviated from this position the images received from cameras also change. Thus, the beam deviation can be evaluated and corrected in accordance with the received images. A beam adjustment method based on the proposed system is disclosed as well.
However, the system described in U.S. Pat. No. 6,587,699 requires the use of additional expensive equipment integrated into the relay system structure and elaborative work of specially qualified staff in the initial antenna beam adjustment procedure and thus, is expensive.
Also known from GB 2459131 is a communication system comprising two transceivers connected to a main antenna and to a search antenna, and a control unit which controls the two transceivers, including activating and selecting operation modes of the transceivers. The mentioned control unit can lead to low efficiency of electronic beam adjustment and possibility of introducing of mistakes due to the ability to only enlarge the radiation pattern beam at the cost of the antenna gain.
Thus, analysis of the above discussed prior art shows that there still exists the necessity of providing a fully electronic fine beam antenna direction adjustment and tracking that would allow fast connection establishing and recovery, and eliminate the need for manual work using qualified staff.